1. Field of the Invention
The present invention relates to a field-sequential type color display system wherein a visual field is composed of a plurality of sub-fields and images in different color are displayed in each of the sub-fields so that multicolor display is effected by mixing colors while taking advantage of the effect of image synthesis along the time base when viewed by human eyes, and a method of driving the same.
2. Description of the Related Art
One type of field-sequential color display system comprises a display unit for emitting light rays of wavelengths in a wide band, capable of supplying display information by light rays of wavelengths varying for respective sub-fields and a variable filter unit for selecting light rays in specific wavelength regions for the respective sub-fields among the light rays of wavelengths in the wideband. In this arrangement, a high density color CRT has become a reality by a combination of a monochrome CRT with a liquid crystal shutter for changing light into the three primary colors: red, green, and blue.
Another type of field-sequential color display system comprises a light source unit capable of emitting light rays of different wavelengths, and a shutter unit for controlling the light rays emitted by the light source unit on the basis of display information, wherein the light source unit is caused to emit light rays in specific colors for the respective sub-fields while controlling the shutter unit in correspondence thereto.
For a color light source, it is used a fluorescent lamp, or a light emitting diode (LED). In particular, as a result of recent development of LEDs emitting blue light, it has become feasible to fabricate the field-sequential color display system by combining LEDs emitting light in the three primary colors with a monochrome shutter of a simple construction. This type of field-sequential color display system, wherein a low cost shutter is utilized without need for a coloring member such as a color filter or the like, is expected to be put to practical application as a display for audio equipment, measuring instruments and the like.
FIG. 7 shows an example of the field-sequential color display system.
The field-sequential color display system is provided with a light source unit 1 composed of a plurality of color light sources which emit light rays of various wavelengths, and which can be controlled independently of one another. The light source unit 1 comprises an LED box 3, wherein a plurality of light emitting diodes (LEDs) 4 for emitting three colors, red, green, and blue, respectively, are arranged as the color light sources, and a diffuser 5, and it is driven by a light source driving circuit 28.
The field-sequential color display system is provided also with a liquid crystal shutter unit 22, operated by the agency of liquid crystal elements, as a shutter unit for controlling transmittivity of the light rays emitted by the light source unit 1. The liquid crystal shutter unit 22 has display segments 26, capable of displaying characters and numbers, and is controlled by a shutter control circuit 29.
The shutter control circuit 29 is connected with the light source driving circuit 28, and the both circuits are synchronously controlled so as to be driven in synchronization with each other.
FIG. 8 is a block diagram of the field-sequential color display system.
The light source unit 1 consists of a red light source R, a green light source G, and a blue light source B, which are lit up by a red light source signal Lr, a green light source signal Lg, and a blue light source signal Lb, respectively, supplied from the light source driving circuit 28.
The liquid crystal shutter unit 22 comprises a plurality of data electrodes 20 and a common electrode 21, and is statically driven by data signals D and a common signal C delivered from the shutter control circuit 29.
FIG. 9 is a waveform chart showing waveforms of respective signals for driving the field-sequential color display system shown in FIGS. 7 and 8, and the optical response characteristic of the liquid crystal shutter unit 22 at a driving voltage of 20V. In this example, for driving the liquid crystal shutter unit 22 by AC power, two fields, f1 and f2, are in use and each of the fields consists of three sub-fields, fR, fG, and fB.
The red light source signal Lr turns on only in the sub-field fR, while turning off in the other sub-fields fG and fB. Similarly, the green light source signal Lg turns on only in the sub-field fG while turning off in the other sub-fields fB and fR. The blue light source signal Lb turns on only in the sub-field fB while turning off in the other sub-fields fR and fG.
The voltage of the common signal C supplied to the liquid crystal shutter unit 22 becomes c1 in the field f1 and c2 in the field f2. In this case, c1 is set at 20V and c2 is set at 0V. The data signals are at either of two voltages, d1 and d2, and, in this instance, it is assumed that d1 is 20V and d2 is 0V.
In the case of adopting an STN liquid crystal panel in normally white display mode for the liquid crystal shutter unit 22, a data signal Dw for displaying white state is in phase with the common signal C, and as the liquid crystals are not supplied with a voltage, the liquid crystal shutter unit 22 is turned into the OFF state. Meanwhile, a data signal Dbl for displaying black state is in opposite phase with respect to the common signal C, and as the liquid crystals are impressed with a driving voltage equivalent to a difference in voltage between the common signal C and the data signal Dbl, the liquid crystal shutter unit 22 is turned into the ON state.
A data signal for displaying one of the primary colors is at a voltage such that the shutter is in the transmitting (open) state only in one of the sub-fields corresponding to the color. For example, a data signal Dr for displaying red color is at a voltage such that the shutter is in the transmitting state only in the sub-fields fR corresponding to red color while it is in the nontransmitting (closed) state in the sub-fields fG and fB.
A data signal Dg for displaying green color is at a voltage such that the shutter is in the transmitting state only in the sub-fields fG corresponding to green color, and a data signal Db for displaying blue color is at a voltage such that the shutter is in the transmitting state only in the sub-fields fB corresponding to blue color.
In the case that the LED box 3 is adopted for the light source unit 1, the emission characteristics of the red light source signal Lr, green light source signal Lg, and blue light source signal Lb can be regarded the same as those of respective LEDs since the response time of the respective LEDs, which are semiconductors, is very fast.
The span of the field f1 is preferably set to not more than 20 ms for obtaining good mixing of colors without causing a viewer to recognize flicker, and accordingly, the span of the sub-fields, fR, fG, and fB, respectively, is set to 5 to 6 ms.
A change from the "closed" to the "open" state of the optical response characteristic Tr of the liquid crystal shutter unit 22 for displaying red is delayed with respect to the data signal Dr for displaying red color by 1.5 to 3.0 ms, equivalent to an OFF response time of the liquid crystal panel. Consequently, the amount of light rays emitted from the red light source is slightly decreased. Similarly, the optical response characteristic Tg for displaying green switches to the "open" state behind the data signal Dg for displaying green color by 1.5 to 3.0 ms, and the optical response characteristic Tb for displaying blue switches to the "open" state behind the data signal Db for displaying blue color by 1.5 to 3.0 ms.
However, since the response time of the liquid crystal panel switching from the "open" to the "closed" state is as fast as 0.1 to 1.0 ms at the driving voltage of 20V or more (depending on the applied voltage), the optical response characteristic Tr when displaying red is completely in the "closed" state in the sub-field fG, with the result that display in red with good chroma is obtained without any mixing of colors caused by the green light source.
Similarly, the optical response characteristic Tg when displaying green, and the optical response characteristic Tb when displaying blue, will cause no mixing of colors caused by the blue and red light sources, respectively, displaying respective colors with high chroma.
Data signals for displaying a plurality or mixture of the primary colors are at a voltage, respectively, such that the shutter is in the transmitting (open) state only in the sub-field corresponding to respective color.
For example, a data signal for displaying bluish green is at a voltage such that the shutters are in the transmitting state in the sub-fields fG and fB, corresponding to green and blue, respectively, while the shutter is in the "closed" state in the sub-field fR. A data signal for displaying purple is at a voltage such that the shutters are in the transmitting state in the sub-fields fB and fR, corresponding to blue and red, respectively. A data signal for displaying yellow is at a voltage such that the shutters are in the transmitting state in the sub-fields fR and fG, corresponding to red and green, respectively.
The conventional field-sequential type color display system of a simple arrangement as described hereinbefore is capable of effecting multicolored display, and can be provided at low cost since a coloring member such as a color filter or the like is not required therein.
However, with the conventional field-sequential color display system using STN (super-twisted nematic) liquid crystal panels in the liquid crystal shutter unit 22, numbers and characters can be displayed by means of eight display segments, but it is difficult to display any character or graphic in dots. The reasons why are described hereinafter.
To achieve display of any character or graphic by means of dot display, the number of display segments needs to be increased through multiplexing driving by use of plural common electrodes.
With an ordinary STN liquid crystal display system, multiplexing driving for performing ON/OFF display by slightly varying effective voltages applied to the respective display segments can be effected by means of a driving method called the voltage averaging method, using the plurality of common electrodes. However, response times in such a case become as long as a hundred to several hundreds of ms due to minimal differences between applied voltages. Hence, it is difficult to adopt the STN liquid crystal panel for the liquid crystal shutter unit of the field-sequential type color display system for displaying any character or graphic by means of dot display.
Then, it is conceivable to use a liquid crystal panel composed of ferroelectric liquid crystals or antiferroelectric liquid crystals, having a memory property, for the liquid crystal shutter unit 22. In this case, for multiplexing driving, it is necessary to divide the span of respective sub-fields into two parts, that is, a writing period of 1 to 2 ms, and a holding period of 4 to 5 ms, and to hold a display state after scanning once with the common electrode while lighting the light source unit during the holding period.
However, the liquid crystal panel composed of ferroelectric liquid crystals or antiferroelectric liquid crystals is not widely used because the gap between liquid crystal cells thereof needs to be controlled to not more than 2 .mu.m, and there is a technical problem of uniformly aligning smectic phase liquid crystals in jelly form, both factors serving to increase the cost.